More tape problems

[Griff Smith]

> I recently inquired about poor performance on a Cipher microstreamer.
> 
> Well, we now have a TU78 (delivered yesterday), and being curious, I
> decideded to play with someone else's TU78 here.  The poor performance
> shocked me.  The is supposed to be a FAST tape drive.
> 
> I was unable to get anything I would call satisfactory using cat, or
> cp (I didn't try dump).  The same TU78 works beautifully on a VMS
> system (swapped the cables), but when used from Unix (4.3BSD), it
> seems to spend more time sitting there than writing.
> 
...
>
> Does anyone have any idea how to get satisfactory performance out of a
> tape drive under Unix?  Help!!
> 
> Jeff
> -------

I don't understand your problem; the TU78 performs very well using
4.3BSD.  Unless something has changed drastically since the pre-release
version that we got from BRL last year (4.2 + planned performance
fixes), the TU78 driver can run the drive at its full rated speed of about
500 Kbyte/second.

It is not clever to use "cat" as a test, however.  The "cat" command
that comes from Berkeley uses stdio to move bytes from input to output
(unless they cleaned this up since I last looked; not done as of
4.2BSD).  This costs about 12 to 14 microseconds per character, which
sets an upper limit of about 80 Kbyte/second.  If you want to see fast
tape performance, look at what you get with "dump".  You should also
try something like

	dd if=some-large-file of=/dev/rmt12 bs=16k

or, even better

	dd if=/dev/rmt12 of=/dev/rmt13 bs=20k

The large block size is important; no tape drive runs anywhere near its
rated speed when the inter-record gap is the same size as the record.

If you use "dd" to convert an EBCDIC tape to ASCII and see poor
performance, don't blame the tape drive.  The standard version of "dd"
takes about 90 microseconds per character to do the byte pushing on a
VAX 11/780, which is about 11 Kbyte/second.  There is a fast version of
"dd" available from the AT&T Toolchest; it does this same conversion
about ten times faster on a VAX 11/780 (the ratio will vary on other
company's machines).
-- 

Griff Smith	AT&T (Bell Laboratories), Murray Hill
Phone:		(201) 582-7736
Internet:	ggs at ulysses.uucp
UUCP:		ulysses!ggs  ( {allegra|ihnp4}!ulysses!ggs )